As a semiconductor light emitting device used for a device such as a small illuminating device, there has been known a semiconductor light emitting device 100 in which, as illustrated in FIG. 8(a), a reflective wall is formed around an LED (light emitting diode) chip 10 which is a semiconductor light emitting element, so as to reflect in an above direction light emitted sideward by the LED chip 10. FIG. 8(b) is a cross sectional drawing taken in C-C′ line of the semiconductor light emitting device 100 in FIG. 8(a).
As illustrated in FIGS. 8(a) and 8(b), in the conventional semiconductor light emitting device 100, the LED chip 10 is formed on a Cu wiring pattern (bonding surface) 16 on a first insulating resin layer 11. A reflective layer 17 made of Ag plating is formed on the Cu wiring pattern. The reflective layer 17 is used for reflecting light from the LED chip 10.
In forming the semiconductor light emitting device 100, in a case of bonding (die-bonding) the LED chip 10 with a bonding surface, Ag paste is used, which is described in Document 1 (Japanese Unexamined Patent Publication No. 311857/2004 (Tokukai 2004-311857; published on Nov. 4, 2004)). However, as illustrated in FIG. 9, if a P-N junction of the LED chip 10 is near a die-bonding surface, a leakage occurs in bonding by Ag paste 101 or the like. Therefore, solder such as AuSn is thinly attached in advance to a die-bonding-side electrode 23 of the LED chip 10 before carrying out die-bonding. At that time, a material for a surface on which the LED chip 10 is to be die-bonded is generally Au in terms of easiness and reliability of die-bonding.
However, in order to emit white light, it is necessary to use a material whose reflectance is high in all ranges of visible light. For example, as described in Document 2 (Japanese Unexamined Patent Publication No. 46137/2003 (Tokukai 2003-46137; published on Feb. 14, 2003)), Ag is used for a surface which reflects light emitted by an LED in a semiconductor light emitting device. However, because of an oxidized layer on an Ag surface, it is difficult to solder the LED chip to Ag. Consequently, as described in Document 2, reflection of light by Ag is impossible on a die-bonding surface. In Document 2, a side wall of an LED package is plated with Ag, but a portion to which the LED chip is die-bonded is not plated with Ag. Emitted light is reflected not only by the side wall but also by a bottom surface. Therefore, if the bottom surface is not plated with Ag, luminosity deteriorates and color tone deteriorates.
Alternatively, in order to obtain a high reflectance by plating a die-bonding surface with Ag, flux is used in die-bonding so as to allow die-bonding on an Ag oxidized film.
Here, functions of flux are the following (1) to (3).    (1) Surface cleaning: to chemically remove an oxidized film on a metal surface (reduction) and make the metal surface clean so that soldering is possible.    (2) Drop of interfacial tension: to reduce a surface tension of molten solder and improve solder wetting.    (3) Prevention of reoxidization: to cover the metal surface in soldering and prevent the metal surface from being in contact with air so as to prevent reoxidization of the metal surface due to heating.
The following explains a conventional method in which die-bonding is performed by use of flux. In order to obtain a high reflectance of Ag on a die-bonding surface while using an LED chip whose P-N junction is near the die-bonding surface in the conventional method, a AuSn solder layer 24 is formed on the die-bonding side electrode 23 of the LED chip 10 as illustrated in FIG. 10(a). Then, as illustrated in FIG. 10(b), flux 102 is applied in advance onto Ag plating, the LED chip 10 is placed on the flux 102, and AuSn and Ag are bonded with each other by reflow soldering.
However, in the conventional method, the LED chip 10 is die-boded with Ag by use of flux, so that a step of applying flux and a step of cleaning flux are necessary. It takes much time to clean flux, and it is difficult to clean flux off a part having complicated structure. In particular, it is impossible to clean flux remaining in a part sealed by solder, which is a main cause of deterioration in reliability of a semiconductor light emitting device.
Further, in a case of using flux, it is necessary to control timing between reaction of flux and melting of solder with reference to a temperature profile of the flux. As such, very minute temperature control is necessary. In addition, remaining flux causes insufficient bonding. Further, as illustrated in FIG. 10(c), when fluidity of flux near the LED chip 10 drops, gas generated from flux near the center of the LED chip 10 cannot go anywhere. Consequently, a void 103 may be generated at an area to be soldered and bonded. At that time, too, insufficient bonding of the LED chip 10 is caused.
Further, in a conventional semiconductor light emitting device, as described in Document 3: Japanese Unexamined Patent Publication No. 49442/2006 (Tokukai 2006-49442; published on Feb. 16, 2006, corresponding US patent publication: US20060022216A1), when die-bonding is performed without flux, die-boding is performed on a whole of Au plating, so that light is reflected by Au. Consequently, color tone deteriorates due to low reflectance of Au in short wavelength range of visible light.